Such automatic assembly machines are also referred to as Die Bonders. A Die Bonder comprises an apparatus referred to as a pick and place system which comprises a bondhead with a chip gripper in order to mount the numerous, uniform semiconductor chips of a wafer, which are located next to each other on a carrier foil, one after the other onto a substrate, eg, a metallic leadframe. A pick and place system is known from the European patent application EP 923 111 with which the bondhead is guided back and forth at high speed between two fixed limit positions. The bondhead slides on a frame. The chip gripper is mounted on bearings and can be moved in vertical direction relative to the bondhead. The frame is lowered for picking the semiconductor chip. As soon as the chip gripper impacts on the semiconductor chip, it is deflected relative to the bondhead whereby the deflection acts against a spring clamped between the chip gripper and the bondhead. The frame is also lowered for placing the semiconductor chip onto the substrate.
A linear guide suitable as a pick and place system is known from the European patent application EP 1 143 487 with which the bondhead can be guided back and forth between any two limit positions.
Two requirements are placed on the mounting process which, with today's systems, can only be fulfilled with great effort. On picking the semiconductor chip, the lowering of the bondhead or chip gripper should take place in the shortest time possible. However, the chip gripper must not impact on the semiconductor chip with too high speed as otherwise the semiconductor chip is damaged or even destroyed by the impact. Also, on placing the semiconductor chip onto the substrate, lowering should take place in the shortest time possible. In addition, the chip gripper should produce a predetermined bond force with which the semiconductor chip is pressed against the portion of adhesive applied to the substrate. While this bond force is relatively low for small semiconductor chips, for the processing of larger semiconductor chips relatively high bond forces are required. The bond force is determined by means of the degree of deflection of the spring clamped between the chip gripper and the bondhead. In order that the picking of the semiconductor chip can take place at a higher lowering speed of the chip gripper, the spring must be relatively soft in order to keep the impact sufficiently low. However, in order that the necessary bond force can be produced for larger semiconductor chips, the spring must be relatively hard as otherwise the deflection necessary to produce the required bond force would be too great.